The present invention relates to a two-component electrophotographic developer to be used for an image forming apparatus using a so-called electrophotographic method, such as an electrostatic copying apparatus, a laser beam printer or the like.
In the image forming apparatus above-mentioned, the surface of a photoreceptor is exposed to light to form an electrostatic latent image on the surface of the photoreceptor. By a developing device, an electrophotographic developer is allowed to come in contact with the surface of the photoreceptor. The powder toner contained in the developer is stuck to the electrostatic latent image, so that the electrostatic latent image is formed into a toner image. From the photoreceptor surface, the toner image is transferred to and fixed on the surface of paper. Thus, an image corresponding to the electrostatic latent image is completed on the paper surface.
As the developer, there is generally used a two-component developer containing a toner and a carrier which is adapted to give an electric charge to the toner by frictional charging and to supply the toner to the electrostatic latent image while adsorbing the toner.
As the carrier, there may be used magnetic particles such as iron powder, ferrite particles or the like. To control the toner electric charge amount and polarity, to improve the dependency on humidity, to prevent the occurrence of filming and to improve the flowability or the like, there is generally used a so-called coated carrier having a core material made of the magnetic particles above-mentioned and a resin coating layer formed on the surface of the core material.
A variety of resins such as thermoplastic and thermosetting resins may be used as the material of the resin coating layer. Of these, a cured body of a silicone resin is suitably used because it advantageously improves the flowability and prevents the filming phenomenon that crushed toner particles or the like stick, as spent particles, to the carrier surface (See Japanese Unexamined Patent Publication No. 186844/1985).
It has also been proposed that a resin coating layer made of any of a variety of resins such as a silicone resin or the like, contains a melamine resin to adjust the electric charging characteristics of the carrier in a suitable range (See Japanese Patent Publication No. 9946/1983, Japanese Unexamined Patent Publication No. 262057/1987 or the like).
However, even though a conventional coated carrier has a resin coating layer of a silicone resin, the carrier particles agglomerate to decrease the carrier in flowability or to generate spent particles. Further, even though the resin coating layer contains a melamine resin, the effect of adjusting the electric charging characteristics has not been sufficient.
On the other hand, the toner is generally produced by mixing a binder resin with a coloring agent and an electric charge controlling agent. The electric charge controlling agent comprises an electron imparting substance or an electron attractive substance to cause the toner to be negatively or positively electrically charged.
However, the electric charge controlling agent is disadvantageously liable to fall down from the toner surface due to its friction with the carrier in the developing step. The electric charge controlling agent falling down from the toner surface contaminates the carrier surface, provoking a decrease in the electric charging ability of the carrier. Further, toner particles from the surfaces of the electric charge controlling agent which fall down, are deteriorated in electric charging characteristics. Further, the electric charge controlling agent is often expensive, thus provoking an increase in toner production cost. Further, the electric charge controlling agent includes a few poisonous examples such as a metallic chelate compound using a metal such as chromium or the like.
Accordingly, it has long been desired to develop a toner containing no electric charge controlling agent. In this connection, Japanese Unexamined Patent Publication No. 280758/1987 discloses a toner in which a composition including a coloring agent and a binder resin as main components, contains a polymer including an acid group and having an acid value of not less than 100.
Further, U.S. Pat. No. 4,504,563 discloses a toner containing a vinyl-type copolymer of which the acid value is in the range from 5 to 100. This Patent discusses an invention proposed with the main object of preventing toner offset. From the description of embodiments of the invention, however, it seems that the invention is directed to a toner containing no electric charge controlling agent.
The toners above-mentioned are preferable in view of production cost or safety, but are not sufficiently satisfactory in view of the control of toner electric charge. This is because no consideration has been made on the relationship between a toner and a carrier exerting a great influence upon the electric charging characteristics of the toner.
It is a main object of the present invention to provide an electrophotographic developer which is not decreased in flowability and which generates no spent particles, thus assuring stable electric charging characteristics.
It is another object of the present invention to provide an electrophotographic developer particularly using a toner containing no electric charge controlling agent, so that the developer is improved in electric charging characteristics and developing characteristics.